The present invention relates to laser devices, and in particular to vertical cavity surface emitting laser (VCSEL) devices such as VCSEL diodes.
Laser devices, such as laser diodes, are being increasingly used in a variety of applications, such as communications and data storage devices. One type of laser diode is the vertical cavity surface emission laser diode (VCSEL), and a typical example is shown in plan view in FIG. 1 and in side cross-sectional view in FIG. 2 of the accompanying drawings. The VCSEL 1 comprises a substrate 7, which carries a lower electrical contact 8. An active region 6 is sandwiched between upper and lower distributed Bragg reflector (DBR) mirror structures 4 and 5, the lower mirror structure being carried by the substrate 7. The mirror structures 4 and 5 provide high reflectivity, for example around 99.5%. The upper DBR mirror structure 4 includes a proton implant area 9. The proton implant area 9 serves to confine current supplied to the device.
An upper electrical contact 3 is provided on the upper surface of the upper mirror structure 4. The upper contact 3 defines an aperture 2 through to the upper surface of the upper mirror structure 4.
The VCSEL 1 of FIGS. 1 and 2 produces a light output which is emitted in the direction of arrow A through the aperture 2. The output light is produced by a lasing effect or laser action in the active region 6 between the two DBR mirrors 4 and 5, as is well known.
One of the main advantages of VCSEL devices is that the light output is produced in a direction perpendicular to the plane of the device. This is in contrast to previous edge emitting laser diodes which emit light in the plane of the device. Thus, VCSEL devices can easily be manufactured into arrays, since a number of devices can be produced on a single semiconductor area, without the need for the devices to be cut from one another. In addition, VCSELs are particularly suited for producing a circular beam of light. Such a circular beam requires little or no further optical processing before application to devices, such as CD ROM drives or communications devices.
However, laser emission in VCSELs can often allow orthogonal or elliptical polarisation states, with incomplete control of the orientation of the axes of polarisation or the polarisation state of individual lasing modes or filaments, which may also vary with the bias current. This can also result in the orientation of the polarisation axes varying from device to device even between adjacent lasers fabricated from the same wafer.
There is much interest in the use of vertical-cavity surface-emitting lasers (VCSELs) for a variety of applications including optical storage, printing and communications. However these applications tend to require high beam quality, in particular with single transverse mode operation where the polarisation state is fixed to a precise direction. Although VCSELs can be formed to generate circular beams and integrated in two dimensional arrays, they suffer from strong optical nonlinearities associated with their high gain operation and strong thermal effects. Such nonlinearities can cause not just strong multimode operation but also filamentary action across the emitting region. This behaviour thus causes beams to be generated with poor far fields and with multiple intensity peaks emitting in poorly defined polarisation states, which can vary with temperature and bias current.
Previous methods for controlling the polarisation state of light emitted from surface-emitting lasers have included the use of anisotropic cavity geometries which may cause distortion of the output beam shape. Introduction of anisotropic stress, gain or loss has also been used for control of polarisation state along pre-defined directions, though complete control is not usually maintained over the entire device operating range.
Temperature detuning of non-degenerate cavity modes has been used for complete polarisation control but this is achieved at the expense of reducing the gain, thereby increasing the device threshold current and power consumption and reducing the available power output. This effect is strongly dependent on the spectral splitting of orthogonal polarisation states, which may be influenced by the residual strain in a device. Therefore an additional technique maybe required to control the spectral splitting if a sufficient gain difference is to be achieved between the two states.
Integration of additional structures with the laser, for example gratings, can provide polarisation control but significantly increase the complexity of the device, and such structures can influence the spatial output characteristics of the laser emission.
According to one aspect of the present invention there is provided a vertical cavity surface emitting laser device having a discontinuity formed within the body of the device, such that, when the device is in use, the direction of polarisation of light emitted from the device is substantially aligned with a boundary of the discontinuity.
The discontinuity may be formed by a void. The void may be provided by an elongated trench which extends into the body of the device from an outer surface thereof.
When the body of the device is of a first material, the discontinuity may be formed by a region of a second material, the second material being different to the first material.
Such a device may comprise a plurality of such discontinuities, the polarisation direction of light emitted by the device when in use being determined by the discontinuities.
A pair of substantially parallel elongate discontinuities may be provided, the direction of polarisation of the emitted light being substantially aligned with the direction of the boundary of the discontinuities which is closest to the light output region.
Alternatively, a pair of substantially mutually orthogonal elongate discontinuities may be used.
According to another aspect of the present invention, there is provided, a vertical cavity surface emitting laser device comprising a substrate, a lower mirror structure carried by the substrate, an active region carried by the lower mirror structure, and an upper mirror structure carried by the active region, and a contact region carried by the upper mirror structure, the contact region defining an aperture through which laser light is emitted when the diode is in use, wherein a discontinuity is defined within the device, such that the polarisation direction of such emitted light is substantially aligned with a boundary of the discontinuity.
In such a device at least one of the mirror structures may be a distributed Bragg reflector structure.
The discontinuity may be provided by a void.
The void may be provided by a trench region which extends through the contact region into the device towards the active region.
The trench region may be formed by etching the device structure, or by laser ablation of the device structure, or by electron beam lithography.
The semiconductor material of the device preferably allows light emission in the 400 nm-4000 nm wavelength range.
According to another aspect of the present invention, there is provided a method of polarising light comprising forming a discontinuity in the body of a vertical cavity surface emitting laser device, such that the polarisation direction of light emitted from the device when in use is substantially aligned with a boundary of the discontinuity.